Method for grinding complex shapes

Abstract

A method of producing a complex shape in a workpiece includes the steps of: i) grinding a workpiece at a maximum specific cutting energy of about 10 Hp / in3·min with at least one bonded abrasive tool, thereby forming a slot in the workpiece; and ii) grinding the slot with at least one mounted point tool, thereby producing the complex shape in the slot. The bonded abrasive tool includes at least about 3 volume % of a filamentary sol-gel alpha-alumina abrasive grain having an average length-to-cross-sectional-width ratio of greater than about 4:1 or an agglomerate thereof. A method of producing a slot in a metallic workpiece having a maximum hardness value of equal to, or less than, about 65 Rc includes the step of grinding the workpiece with a bonded abrasive tool at a material removal rate in a range of between about 0.25 in3 / min·in and about 60 in3 / min·in and at a maximum specific cutting energy of about 10 Hp / in3·min.

Description

BACKGROUND OF THE INVENTION[0001]A re-entrant shape is a form which is wider at the inside than it is at the entrance (e.g., a dovetail joint). Turbine components, such as jet engine, rotors, compressor blade assembly, typically employ re-entrant shaped slots in the turbine disks. The re-entrant shape is used to hold or retain turbine blades around the periphery of turbine disks. Mechanical slides, T-slots to clamp parts on a machine table also use such re-entrant shaped slots.[0002]This type of form cannot generally be created by grinding with a large diameter wheel operated perpendicular to the surface of the part because it would be impossible for the wheel to enter the wider part of the form without removing the narrower part of the form. Typically, broaching or milling has been used in the aerospace industry to produce such a complex shape. Broaching a re-entrant shape, however, is costly partly due to high tooling costs, such as expensive machinery, set-up costs, tooling regri...

AI Technical Summary

Benefits of technology

[0005]It has now been discovered that bonded abrasive tools made with a filamentary sol-gel alpha-alumina abrasive grain or an agglomerate thereof, can produce effectively a slot for a re-entrant shape in a workpiece, in particular in a hard-to-grind metallic workpiece, with a high metal removal rate. It also has now been discovered that the slot formation process followed by a complex-shape (e.g., re-entrant shape) formation process with at least one mounted point tool can produce a desired complex shape with a good surface finish in a relatively short process time as compared with that of the conventional milling or broaching process. These processes can be carried out utilizing a water-based coolant in place of traditional oil coolants. Based upon these discoveries, slot formation processes with a bonded abrasive tool to remove the bulk of material for producing a complex shape, and methods for producing a complex shape in a workpiece that employ such a slot formation process are disclosed herein.

[0007]In another embodiment, the present invention is directed to a method of producing a slot in a metallic workpiece having a maximum Rockwell hardness value of equal to, or less than, about 65 Rc. The method comprises the step of grinding the workpiece with a bonded abrasive tool at a material removal rate in a range of between about 0.25 in3 / min·in (about 2.7 mm3 / sec / mm) and about 60 in3 / min·in (about 650 mm3 / sec / mm) and at a maximum specific cutting energy of about 10 Hp / in3·min (about 27 W·s / mm3). Alternatively, the method comprises the step of grinding the workpiece with a bonded abrasive tool at a material removal rate in a range of between about 2 mm3 / sec / mm and about 700 mm3 / sec / mm and at a maximum specific cutting energy of about 30 J / mm3. The slot formation processes of the invention can remove the bulk of material, minimizing the amount of material to be removed in the complex shape grinding processes with a mounted point tool. The slot formation processes of the invention can also reduce the arc of contact of the mounted point tool. In particular, the slot formation processes of the invention, employing a bonded abrasive tool that includes a filamentary sol-gel alumina abrasive grain, have outstanding performance with high metal removal rates and at relatively low specific cutting energies. The low specific cutting energies in turn minimize heat generation in the grinding zone, thus reducing risk of metallurgical damage to workpieces.

[0008]The methods of the invention for producing a complex shape that employs such slot formation processes can significantly reduce process costs compared with the conventional processes (e.g., milling and broaching) without compromising surface-finish quality and / or structural integrity of the resultant complex-shaped work product.

Problems solved by technology

This type of form cannot generally be created by grinding with a large diameter wheel operated perpendicular to the surface of the part because it would be impossible for the wheel to enter the wider part of the form without removing the narrower part of the form.

Broaching a re-entrant shape, however, is costly partly due to high tooling costs, such as expensive machinery, set-up costs, tooling regrinding costs and slow material removal rates.

However, this requires frequent tool changes and re-sharpening of dulled cutting edges, which is cost and time intensive.

Milling processes are generally very slow, especially in machining difficult-to-machine materials, such as Inconel™ nickel alloy, which is typically used for re-entrant shaped turbine disks of aeroengines.

Although high speed milling can be conducted to achieve high efficiency, under such high speeds, fracture of the cutting edge of milling tools commonly occurs, often leading to imbalance, tool fracture and failure of the process.

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